The present invention relates to measurement and control systems which are applicable to various types of industrial and scientific processes, and is particularly concerned with a multiple-loop temperature controller for use in automated blood culturing systems and other types of medical and biological testing systems requiring precise temperature control.
In the fields of industrial and scientific process control, there are many situations in which it is necessary to maintain precise control over one or more parameters, such as temperature. Typically, this is done by continuously measuring the parameter of interest and maintaining a desired set point value of the parameter using some type of closed-loop control system, such as a proportional integral derivative (PID) controller. In automated blood culturing systems, for example, the parameter of interest is temperature and the control system is designed to maintain a specific set point temperature (usually 35.degree. C.) which is favorable to the growth and detection of bacteria and other microorganisms.
In the BACTEC.RTM. automated blood culturing system developed by Becton Dickinson and Company, the assignee of the present invention, individual blood samples to be cultured are placed in vials which are coated internally with a composition that fluoresces in the presence of carbon dioxide (CO.sub.2) produced by microbes. The vials are placed in an enclosed cabinet or housing which is heated to a predetermined temperature (preferably 30.degree. C.) somewhat lower than the desired vial temperature, and are held in racks which are separately heated to produce the desired 35.degree. C. vial temperature. The vials are received in individual cavities or wells formed in the racks, and are agitated by continuous rotation to promote microorganism growth. Each cavity contains a light-emitting diode (LED) and a cooperating photodiode detector, with suitable filters to restrict the LED and photodiode to specific wavelengths of light. The LED serves as an excitation source for causing the CO.sub.2 -responsive material at the bottom of the vial to fluoresce, and the photodiode detector is used to detect this fluorescence to provide an indication that a certain level of bacterial or fungal growth has occurred in the vial. By detecting microorganism growth in this way, the blood culturing system can operate automatically under computer control, without the need for continuous human supervision. In the BACTEC.RTM. system, each cabinet is designed to accommodate up to six racks holding up to 240 individual vials, and several cabinets can be monitored by a single computer.
The accuracy of an automated blood culturing system of the type described above depends to a great extent on maintaining a precisely controlled temperature at each of the test vials. This is done by providing each rack with a separate heating device and temperature sensor, and by connecting the heating devices and temperature sensors to a closed-loop temperature control system that operates under microprocessor control. Rather than utilizing a separate control system for each rack or loop, a multiple-loop control system is provided in which the microprocessor operates in a time-shared manner to control all of the temperature loops essentially simultaneously. The processing speed of currently available microprocessors is sufficiently fast that precise temperature control can be achieved even when a relatively large number of racks or loops are being controlled.
In order to allow for time-shared operation of a temperature controller, some means must be provided to obtain temperature readings from the individual temperature sensors on a periodic or cyclical basis. When precise temperature control is required, the cycle time must be fairly short so that any temperature excursions can be quickly detected and corrected. However, time-shared operation with multiple sensors can present difficulties when certain types of temperature sensors are used, such as platinum resistance temperature devices (RTDs). RTDs are capable of detecting temperatures with great accuracy, but they are passive devices and require a power source in order to produce an output representing the sensed temperature. This can be achieved by connecting each RTD into a resistive bridge circuit and using the output voltage of the bridge to represent the sensed temperature. However, this arrangement is disadvantageous in a multiple-sensor system, not only because a large number of components are required for the various bridge circuits, but also because slight differences among nominally identical components can significantly affect the accuracy of temperature measurement. Moreover, the bridge output from each sensor must be converted to digital form for use by the microprocessor, and this has been done by using a specific type of analog-to-digital (A/D) converter, known as a voltage-to-frequency (V/F) converter, to convert the bridge voltage to a frequency value which can then be converted to a digital temperature value by the microprocessor. Since it is not practical to provide a separate V/F converter for each temperature sensor, a multiplexer is used to switch this component among the various sensors. Unfortunately, the multiplexer itself can introduce temperature measurement errors due to leakage currents within the multiplexer.
In general, calibration can be used to eliminate or reduce certain types of errors which can occur in temperature measurement and control systems. However, if calibration occurs only during the initial power-up interval, some types of errors (such as temperature drift of component values) will not be corrected. Moreover, calibration is effective only to the extent that the same components and interconnections used during the actual measurement operation are also used during the calibration operation, so that any errors contributed by these components and interconnections will be taken into account. When some or all of the components and interconnections used to calibrate the measurement device are different from those used during the measurement operation, the potential for inaccuracy remains.
As noted previously, voltage-to-frequency (V/F) converters have been used previously to convert the analog voltage signals from RTD sensors to frequencies, which are in turn converted to digital temperature values for processing. Although this is a useful method for converting analog RTD outputs to digital temperature values, certain newer types of analog-to-digital (A/D) converters provide much greater conversion accuracy than V/F converters. In particular, sigma delta or charge balancing A/D converters provide higher resolution and lower nonlinearity error than V/F converters. Sigma delta A/D converters also contain internal digital filters which provide excellent filtering of line frequency noise that may occur on RTD wire runs. Unfortunately, the relatively long settling time of the digital filter in a sigma delta A/D converter makes it difficult to use such a converter in a time-shared measurement or control system, since the data does not become available at the outputs of the A/D converter quickly enough to maintain a rapid cycle time for the system as a whole. Ideally, it would be desirable to overcome this obstacle so that sigma delta A/D converters, with their attendant advantages of high resolution, low nonlinearity error and superior filtering, can be employed in time-shared measurement and control systems without requiring unduly long cycle times.